Light naphtha conversion process



July 15, 1969 G. Hovl-:STREYDT ET AL 3,455,813

LIGHT NAPHTHA CONVERSION PROCESS Filed July 26, 1966 2 Sheets-Sheet 1 July 15, 1969 G. HOVESTREYDT ET AL 3,455,813

I LIGHT NAPHTHA CONVERSION PROCESS Filed July 26, 1966 2 Sheets-Sheet 2 United States Patent 4O 3,455,813 LIGHT NAPI-ITHA CONVERSION PROCESS Gerrit Hovestreydt, Beek, Limburg, and Johan D. Logemann and Kees Delcour, Geleen, Netherlands, assignors to Stamicarbon N.V., Heerlen, Netherlands Filed July 26, 1966, Ser. No. 567,940 Claims priority, application Netherlands, July 26, 1965, 6509667 Int. Cl. Cg 37/10, 35/08 U.S. Cl. 208-65 3 Claims ABSTRACT OF THE DISCLOSURE The recovery of hydrocarbon products from light naphtha oil is discussed, with .review of various known processes. A new process is described for treating light naphtha oil to provide a liquid fraction with high aromatic content and a gaseous fraction rich in 2 4 carbon atom hydrocarbon. These fractions result from a process in which there is rst a catalytic hydro-forming treatment of the oil, and thereafter there is a catalytic cracking step.

Summary of the invention The present invention .relates to a process for preparation of saturated aliphatic hydrocarbons containing 1-4 C atoms, in combination with aromatic hydrocarbons (benzene and its homologues), from light naphtha having a Vboiling range of 40 to 11G-180 C.

Background of the .invention Various processes have been proposed for the preparation and removing of various hydrocarbon products from the thermal cracking products obtained from light naphtha oil. Light naphtha is a well known feedstock material which may be processed to provide products of valuable aromatic hydrocarbons in other products. However, commercially suitable processes to obtain such valuable products have not really been developed for there are severe disadvantages in the various prior processes which have been used, rendering them unsuitable as a manufacturing method to produce the said aromatic hydrocarbons, etc.

It is, for instance, well known that the hydrocarbons present in light naphtha having a yboiling range of 40 to 11G-180 C., in particular those hydrocarbons containing from 5 to 7-9 carbon atoms, can, by thermal cracking at temperatures of 'ZOO-900 C., be principally converted into a major proportion of gaseous unsaturated hydrocarbons having 2-4 C atoms, along with a minor amount of methane and hydrogen,

Depending on the composition of the starting material and on the process condition, such a cracking process converts, for instance, -'25 percent of the naphtha into ethylene, -55 percent into methane, propylene, and unsaturated C., hydrocarbons, and, in addition, 20-30 percent into liquid hydrocarbons. The latter fraction is the so-called cracked naphtha, which in most cases has a higher aromatic content than the original naphtha (all gures referring to parts by weight).

The liquid hydrocarbons obtained in rsuch a process have considerable value as fuel for petrol engines, due to their higher aromatic content.

However, this liquid fraction is not well suited as a starting material for the commercial recovery` of pure aromatics as ultimate products, because of its relatively low aromatic content and the presence of unsaturated hydrocarbons.

Of the gaseous products obtained from the cracking process, ethylene and butadiene are the most valuable as starting materials for chemical synthesis processes. In

3,455,813 Patented July l5, 1969 ICC general, the other gases will yield no more than their simple fuel value.

In another lknown process (see e.g., the British patent specification 935,681), the light naphtha feed material is first cracked by heating, whereby a large portion of it is converted into methane, ethylene, gaseous unsaturated 3-4 carbon hydrocarbons and an aromatic-containing liquid fraction (cracked naphtha). The latter fraction, after the gases are separated from it, is subjected to a catalytic treatment with hydrogen, carried out under pressure and under relative mild conditions of temperature, so as to convert the olefines content thereof into saturated alkanes.

VThe resulting naphtha is then suitable to use as a starting material for the extraction of armomatics therefrom, while subsequently, after the aromatics have been recovered, the remaining alkane-containing liquid is recycled to the thermal racking step.

This process is complicated, however, and has the disadvantage that the aromatic content of the finally resulting cracked naphtha is still so low that the aromatics can only be effectively recovered by extraction, which is a costly method.

In still another known process (see eg. U.S. Patent specification No. 2,143,472) for the recovery of aromatics and oletins of low boiling point, the starting material is an oil distillate containing naphthenes. This material is subjected, in the vapor phase, to a catalytic treatment whereby the naphthenes are converted to aromatics, the reaction mixture being subsequently subjected to a noncatalytic cracking process in order that the parallins having a higher boiling point than the aromatics and any remaining naphthenes as well, may be converted to olefines and parans of lower boiling point. The reaction mixture is thereafter separated, by rectification, into a tar fraction, a fraction of aromatics, fractions consisting of parans and olenes, and gaseous hydrocarbons.

To recover pure aromatics in this process, the aromatic fraction obtained must again be subjected to an extraction treatment, to effect a separation between the nou-aromaics still present in the aromatic fractionwhich do not dissolve in the extraction agent-and the aromatic properwhich form a solution with the extraction agent.

Object of the invention The present invention provides as the principal object a process in which the light naphtha is so treated as to yield (l) a liquid fraction having a high aromatic content, this content being so high that the aromatics can Ibe recovered from the fraction by a simple separatory process such as rectication, which is much cheaper than extraction, and (2) a gaseous saturated hydrocarbon fraction containing for the greater part 2-4 carbon atoms. This gaseous hydrocarbons fraction is extraordinarily wellsuited for use as a starting material for the preparation of ethylene by thermal cracking, with the by-product formation of only a small amount of less valuable material, such as propylene, in addition to the ethylene.

The process according to the invention has the added advantage of flexibility as regards the products to be recovered. That is, by change of the reaction conditions, the relative amount of the aromatics and gaseous hydrocarbons fractions may be singly altered, as desired GENERAL DESCRIPTION OF THE INVENTION The effects referred to above are obtained, according to this invention, in that the starting material, light naphtha with a boiling range of 40 to 180 C., is subjected to a treatment with hydrogen at certain elevated temperature and pressure and in the presence of a dehydrogenation catalyst, so that the aromatics originally present in the naphtha are preserved, While also a considerable amount of additional aromatics and, simultaneously, some 1-4 C atom saturated hydrocarbons are formed. After separation of the resulting gaseous and liquid hydrocarbon fractions there is obtained a liquid fraction rich in aromatics (at least 70 percent by weight) and otherwise consisting mainly of saturated C5 hydrocarbons.

The catalytic hydrogen reaction, under pressure, as provided and practiced according to this invention, essentially involves two reactions that are separately known in themselves.

Firstly, there is the so-called catalytic hydroforming treatment of the naphtha, which process increases the aromatic content of the oil. This occurs, for instance, by the dehydrogenation of the 6-8 C atom naphthenes content of the feed and by the isomerization of cyclic naphthenes with 5 C atoms in the ring to naphthenes with 6 C atoms in the ring, followed by dehydrogenation and by cyclization (with loss of hydrogen) of parain components to aromatics. Secondly, there is a catalytic cracking process taking place with hydrogenation of the higher parans to lower boiling saturated hydrocarbons with 1 4 C atoms.

The conditions under which the hydrogen treatment of the light naphtha takes place according to the invention are different from those under which the hydroforming of naphtha is generally performed, so that the iinal result is also different.

Thus, the object of the hydroforming process is the formation of aromatics, in particular toluene and xylene, and isomerization of the liquid hydrocarbons. The process thus gives rises to release of free hydrogen, but is so controlled that the amount of gaseous hydrocarbons formed is as small as possible.

In the hydrogen treatment according to the invention aromatics are also formed, but, in addition, a large proportion of the liquid non-aromatic hydrocarbons present in the starting material is converted into gaseous saturated 1-4 C atom hydrocarbon material, with no release of hydrogen. By contrast, the hydrogen treatment in general involves the consumption of a little hydrogen, c g. -3 percent by weight, calculated to the amount of naphtha treated, depending on the composition of the original naphtha and the ratio between the aromatics and the gaseous products formed.

The more gas that is formed, the greater the hydrogen consumption will be.

In addition, if in the hydrogen treatment a higher temperature is applied, the ratio of benzene to its homologs in the resulting liquid containing aromatics can be influenced in favor of benzene.

The desired conversions are effected with the use of normal dehydrogenation catalysts, i.e. of such metal catalysts as Cr, Mo, Pt, Pd, or other noble metals, applied to a carrier.

Preferably very active platinum catalysts are used, with the platinum applied, in an amount of bet-Ween 0.1 and 2 percent by weight, to a carrier material consisting mainly of alumina or silica or a mixture of alumina and silica.

The conditions of temperature and pressure, and the hydrogen-to-naphtha ratio in the hydrogen treatment, are so chosen that the aromatic content in the resulting liquid fraction will be at least 70 percent by weight, this fraction otherwise consisting mainly of saturated C hydrocarbons. At the same time, half or more of the original starting material is converted into gaseous saturated C1-C4 hydrocarbons.

The pressures needed to achieve this end may vary, for instance, from about to 90 atm., preferably from about 25 to 60 atm. The molar hydrogen-to-naphtha ratio may be from about 4 to 20, while final temperatures of, for instance, about 550 to 600 C. can be used. The light naphtha may be passed over the catalyst at the rate of, say, about 0.5-10 volumes (liquid) per volume of catalyst and per hour.

At the start of the hydrogen treatment the temperature may be lower, as the formation of aromatics will proceed at temperatures of about 450 to 525 C., but in order to effect the equally desirable conversion of the greater part of the paratiins that cannot be converted to aromatics to the desired gaseous saturated C1-C4 hydrocarbons, it is then necessary to raise the temperature by about 10D-50 C. towards the end of the hydrogen treatment.

It will be advantageous, therefore, to carry out the hydrogen treatment in successive stages, each stage working with a different type of catalyst, if so desired, with a relatively low temperature and/or pressure being mainl tained in the irst few stages, anda higher temperature and/or pressure in the nal stages. The hydrogen treatment may be carried out with the catalyst mass present either as a fluid bed or as a xed bed.

The process according to the invention has the advantage of yielding a liquid with a high content of valuable aromatics and doing away with the necessity of recovering these aromatics by extraction, since a simpler, less costly separation, viz. by rectification, will suflice.

A separation process of this kind, in which use is made of distillation, may be carried out, e.g. in a iirst stage, yielding a top product consisting of benzene and nonaromatic hydrocarbons and a bottom product, which in a following rectication can be separated into toluene as the top product and xylene contaminated with ethyl benzene as the bottom product.

The mixture obtained as the top product in the first rectication can be used for the recovery of pure benzene in a known way by subjecting it to azeotropic distillation with, for instance, acetone as an auxiliary liquid.

In order to produce ethylene, the saturated gaseous hydrocarbons obtained in the hydrogen treatment are subjected to thermal cracking. Prior t0 the cracking proper, methane and hydrogen, which do not contribute to the formation of ethylene, can be separated from these gases.

The starting material used in the hydrogen treatment according to the invention need not be largely freed of sulphur compounds.

Light naphtha having a sulphur content of, say, 1000 p.p.m. may be processed as such. Although slightly less aromatics will then be formed, the amount of gaseous hydrocarbons that can be cracked to produce ethylene will be higher.

Description of the drawings The processes according to the invention will be explained with reference to FIGURES I and II, which show process diagrams.

In the process according to FIGURE I, naphtha is supplied through conduit 1 to reactor R, in which the hydrogen treatment under pressure is effected.

The reaction mixture formed in sent, through conduit 2, to a separator S1, in which the gases are separated from the liquid fraction subsequent to cooling.

Through conduit 3 the liquid fraction is passed into a stripper column D, the top product from which consists of the still-dissolved gases, and the bottom liquid product is then passed to a `following rectifying column D2 through a conduit 4. In this column a separation is made between benzene and non-aromatic, mainly C5, hydrocarbonsdischarged through conduit 6-on the one hand, and the higher-boiling toluene xylene on the other hand. The toluene-xylene mixture is sent, through conduit 5, to a rectifying column D3, the top product from which, owing oft through conduit 7, is toluene, and the bottom product, removed through conduit 8, the xylene, which always also contains some ethyl-benzene.

The mixture of benzene and parains removed as the top product from column D2 can be separated in the usual way, by distillation, with e.g., acetone as an auxiliary liquid to recover pure benzene.

The gaseosu mixture of hydrogen and saturated C1-C4 hydrocarbons to be discharged from separator S1 is sent,

through conduit 9, to a separator S2, in which hydrogen is separated from the hydrocarbons by cooling. This hydrogen, restored to the proper pressure by compresser P, if necessary, is recycled to reactor R.

The hydrocarbons to be discharged from separator S2, and also the hydrocarbons recovered at the top product in distillation column D1, are passed, through conduits and 11, respectively, to cracker K1, Where the ethane, propane, and butane are subjected to an non-catalytic thermal cracking process in which ethylene is formed; the resulting reaction mixture is sent to a gas separator S3 through conduit 12.

The ethylene produced is discharged through conduit 13, and ethane not converted to ethylene is sent to ethane cracker K2 through conduit 14. The gas mixture obtained in this crackng is again sent to gas separator S2, through conduit 16. Through discharge conduits 15, of which only one is shown, residual gases (a mixture of, chiefly, methane, propylene, and propane) are removed from the system. If desired, these residual gases can be returned to a thermal cracker, either before or after they have been separated into their constituents.

The process according to FIGURE II differs from that according to FIGURE I in that hydrogen treatment is carried out in several reactors arranged in series, which are designated as R1 and R2, and in that the gaseous mixture of saturated C1-C4 hydrocarbons discharged from separator S2 and stripper D1 through conduit 11 is rst sent to a gas separator S4 for the removal of methane, so that the load on the crackers will be lighter. The methane fraction, which always contains some components that can be cracked to produce ethylene, is then sent to gas separator S3 through conduit 18; the C2-C4 fraction to be discharged from separator S4 is fed to cracker K1 through conduit 17.

Reactor R1 may contain a catalyst more specifically suited for the formation of aromatics, for instance Pt on A1203, Whether or not promoted by a small amount of a halogen (Cl, F), whereas reactor R2 may contain a catalyst more specically adapted to the destructive hydrogenation of the paraflins, for instance Pt on A1203, promoted by a small amount of alkali or alkaline earth metals, or Pt on SiO2. It is also possible to raise the temperature in reactor R2 slightly above the level maintained in reactor R1, for instance 50 C. above this level.

As the dehydrogenation reactions are endothermic, the reaction temperature or the temperature variation in reactor Rl can be controlled as desired by the supply of heat, Whereas in reactor R2 generally cooling will have to be applied, to remove heat generated in the destructive hydrogenation of the parains and oleines.

The procedures represented in FIGURES I and II show only theprincipals of operation. Of course, in practice use is made in the hydrogen treatment of several reactors, these being taken out of operation periodically in order to regenerate the catalyst which Will have become inactive in the reactor, e.g. owing to the deposition of carbon.

Furthermore, in general it will be found necessary to apply hydrogen mak-up, as in the hydrogen treatment hydrogen is consumed in an amount of 0-3 percent of the feed.

The following examples will give some idea of the results that can be achieved by means of the process according to the invention.

Example I In apparatus as represented in FIGURE I a light naphtha feed material with a boiling range of 40-160 C. was rst led over a platinum-on-alumina catalyst (0.6 percent by weight of Pt, 0.66 percent by weight of Cl), at a space velocity of l litre of naphtha per litre of catalyst and per hour, together with a vefold amount of hydrogen (calculated as grammolecules), at a temperature of 656 C. and at 30 atm.

For every grams of feed, this hydrogen treatment yielded 68 grams of gaseous C1-C2 hydrocarbons, viz.:

Grams CH4 11 C2H6 20 CBHE 26 CifHlo which, together with steam, were fed to a thermal cracker Kl and were there cracked at a temperature of 800 C. The cracked reaction mixture was passed to the gas separator S2, from which 21.6 grams of ethane were sent, through conduit 14, to ethane cracker K2, to be again subjected to a cracking treatment, carried out at a temperature of 820 C.

Eventually, the gas separation yielded, per 100 grams of the original naphtha, 29.2 grams of ethylene, plus residual gases consisting of a hydrogen-methane fraction (23.9 grams), a propylene-propane fraction (11.5 grams), and a butylene-butane fraction (2.9 grams). The liquid fraction (34 grams) obtained in the hydrogen treatment, which had an aromatic content of 87.6 percent, can be separated into:

Grams Benzene 5.3

Toluene 13.1 Xylene and ethyl-benzene 11.4

Non-aromatic, mainly consisting of saturated C5 and a small amount of (l5-{- hydrocarbons 4.2

If the original naphtha was cracked direct, i.e. without having undergone the hydrogen treatment, this would have resulted in the production of:

Grams Ethylene 23.5 Methane-l-hydrogen 15.5 Propylene-l-propane 18.5 C4 hydrocarbons 11.5

Cracked naphtha with an aromatic content of 43 percent 26 Example II In the same way as described in Example I a naphtha with a boiling range of 40-116 C. was processed.

After the hydrogen treatment, every 100 grams of the original naphtha yielded 64 grams of gaseous C1-C4 hydrocarbons, viz:

Grams CH4 10 C2H6 19 C3H2 24 C4H10 11 After the thermal cracking these yielded 27.9 grams of ethylene, and residual gases consisting of a hydrogenmethane fraction (21.7 grams), a propylene-propane fraction (10.9), and a butylene-butane fraction (2.9 grams).

The liquid fraction (37 grams) obtained after the hydrogen treatment, which had an aromatic content of 89 percent, could be separated into:

. Grams Benzene 9.3

Toluene 16.9

Xylene-l-ethyl-benzene 6.7

Non-aromatics, ymainly consisting of C5, and for the remaining part of C5+hydrocarbons 4.1

If this naphtha had been subjected direct to thermal crac-king, without having undergone the hydrogen treatment, the resulting products would have been:

Grams Ethylene 24.5 Methane-t-hydrogen 14 Propylene-l-propane 20.5 C4 hydrocarbons 12 Cracked naphtha with an aromatic content of 20 percent 23 What is claimed is:

1. A process for the simultaneous conversion of light naphtha oil feedstock having a boiling range of at least about 40 C. and up to a range between about 110-180 C. into a fraction composed of saturated aliphatic hydrocarbons containing from 1 to 4 carbon atoms and a fraction composed of monocyclic carbocyclie aromatic hydrocarbons consisting essentially of benzene and lower alkyl homologues thereof, which process comprises h'ydroforming and cracking said naphtha by subjecting said naphtha to a treatment in two stages with free hydrogen gas at an elevated temperature in the range of 450 to 600 C. and under an elevated pressure of between about 15 atm. and 60 atm., said rst stage being carried out in the presence of a catalyst composed of from about 0.1 to about 2 percent by weight of platinum metal on a carrier of A1203 promoted by halogen, and the said second stage being carried out in the presence of a catalyst containing Pt on a carrier of A1203 promoted by alkali or alkaline earth metals, or Pt on a carrier of SiO2, wherein the temperature at the end of said treatment is between about S50-600 C., while maintaining a molar ratio of hydrogen to naphtha at a value between about 4-20, and while passing said naphtha over the catalysts at the rate of about 0.5-10 liquid volumes per volume of catalyst per hour,

such that at least about one-half of said naphtha is converted to gaseous saturated hydrocarbons containing from 1 to 4 carbon atoms, and the remaining part of said naphtha feedstock is converted substantially into a liquid reaction product having a content of said aromatic compounds of at least about percent by Weight, the remaining portions thereof consisting essentially of saturated hydrocarbons of ve carbon atoms.

2. The process of claim 1 wherein the said liquid containing said aromatic compounds is subjected to a rectilication procedure to recover the aromatic compounds therefrom.

3. The process of claim 1 wherein the said saturated gaseous hydrocarbon product is subjected to a thermal cracking process to form and recover ethylene therefrom.

References Cited UNITED STATES PATENTS 10/ 1959 Schneider et al. 208-138 8/1965 Evans 208-137 U.S. C1. X.R. 208-136, 138, 139 

